According to Brownstein-Tarr theory and using the series expansion method, the Bloch equations controlled by diffusion effect are solved for double-phase pore space of sphere-capillary model saturated with oil and water, and the theoretical relaxation computational formulas for the sphere-capillary model are obtained. The result shows that the relaxation process of oil is only related to oil saturation and isn't associated with pore structure on the condition of wetness. A great deal of computation simulations were carried out based on the theoretical computational formulas, and the numerical simulation results show that the main relaxation process of sphere or capillary is a single exponential function and the other relaxation processes are negligible. In other words, the relaxation of sphere-capillary model is approximatively a double exponential descending process.